Wheats are commonly classified, according to varieties and their natural tendency and according to the protein content of the grain, into soft wheats and hard wheats. Soft wheats are divided into
fodder wheats, which are unsuitable for bread-making, PA1 wheats which are suitable for bread-making (12% minimum of proteins), and PA1 wheats of first-class quality or improved wheats (13.5% minimum of proteins).
Hard wheats are characterized by a 14.5% minimum of proteins.
The baking quality of wheats is closely related to the variety but also to highly significant external factors such as "the year effect" and the nitrogen fertilization, so that farmers are never assured of seeing their batch of cereals accepted for such or such a use.
The yield to the hectare of a cereal field is a matter of fertilization of the soil. It is, moreover, known that nitrogen fertilization of wheat crops is reflected by an increase in the protein content of the grain. It is known, for example in Champagne, a region of France where yields are generally high and protein richness low, that a supplement of 80 nitrogen units supplied to the soil increases the protein content by approximately 1%. This is assessed by the apparent use coefficient (AUC) of the nitrogen which is calculated by the formula ##EQU1## wherein .DELTA.N is the difference in nitrogen between an unfertilized control and the sample originating from a fertilized plot at a given dose or the difference between samples originating from two plots which have been fertilized in different ways, or alternatively ##EQU2##
For an application, in fertilization, of ammonium nitrate to the soil at the "ear swelling" stage at a dose of 40 kg N/ha, the AUC is of the order of 50% to 60%. The AUC becomes better as the nitrogen application to the soil is made later (for example, by application at the stage of swelling of the ear), provided, however, that the application is followed by rain, which is not always the case, and, in any event, the AUC thus obtained reaches 80% at the very most.